The amino acid selenomethionine has been used in x-ray crystallography as a label to determine the phase in place of heavy metal derivatives. However, in some cases, the selenomethioninecontaining protein crystals give poor diffraction patterns. One hypothesis for this phenomenon is that selenomethionine is oxidized more easily than methionine. To examine this hypothesis, we have used matrix-assisted laser desorption mass spectrometry to determine the oxidized products generated by hydrogen peroxide oxidation. To better characterize the sensitivity to oxidation for selenomethionine residue in peptide, we employed reverse-phase HPLC to quantitatively analyze the oxidized products and on-line HPLC/ESI mass spectrometry to identify the products. By this means, we have characterized the oxidation properties of selenomethionine residue in peptide. A paper describing this work is in preparation.